The ionosphere is defined as the layer of the Earth's atmosphere that is ionized by solar and cosmic radiation. It lies 75-1000 km (46-621 miles) above the Earth. During the day, energy from the Sun ionizes, or strip the atoms in this area of one or more of their electrons to create positively charged atoms. The ionized electrons behave as free particles. Only half the Earth's ionosphere is being ionized by the Sun at any time (e.g., during daylight hours). During the night, without interference from the Sun, cosmic rays ionize the ionosphere, though the effect is not a pronounced as during the day. Thus the ionosphere is much less charged during the nighttime due to the lack of sunlight, but is still present due to the effect of cosmic rays. The ionosphere has major importance to us because, among other functions, it influences radio propagation to distant places on the Earth, and between satellites and Earth. The ionosphere is a very dynamic region in that the distance of the bottom layer and the top layer above the Earth varies and changes throughout the day. Furthermore, the electron density of the ionosphere varies. In addition to this, local variations in the ionosphere can travel in a manner similar to waves through the ionosphere. These local variations can span an area as small as a few miles and are difficult to predict.
The ionosphere has major importance to us because, among other functions, it influences radio propagation to distant places on the Earth, and between satellites and Earth. Because of the influence of the ionosphere on radio propagation between satellites and the Earth, timely and accurate modeling of the ionosphere is important in the field of satellite navigation. More specifically, the ionosphere slows down radio signals from orbiting navigation satellites, resulting in a timing error causing the pseudorange to appear to be longer than it really is, and so the precision in determining the location of a navigation receiver is diminished. Also, because the Sun's angle relative to the ionosphere affects the amount of energy available for ionizing atoms, the diurnal (e.g., time of day) and seasonal effects on the ionosphere are important variables to model. Additionally, as described above, local variations, which have a spatial variation and temporal duration that only extend over short times and distances, make it difficult to create accurate models of the ionosphere.